Burners which operate in high temperature furnaces are cooled in order to preserve the structural integrity of the burner components and to retard the oxidation rate of hot metallic surfaces. In conventional burners using ambient air as the oxidant, adequate cooling of the burner is generally provided by the combustion air.
Recently the use of oxygen or oxygen-enriched air has been gaining prominence as an oxidant for burners because its use is more energy efficient and less pollution generating than the use of air. However, use of oxygen or oxygen-enriched air as the burner oxidant has resulted in a number of problems with conventional cooling systems designed to cool a burner which uses air as the oxidant.
First, as is well known, oxygen or oxygen-enriched air typically produces a hotter flame than that produced by air. Thus oxygen burners are exposed to higher heat flux from the flame. A second problem with oxygen burners results from the fact that the volume of the oxidant required to burn a unit amount of fuel is reduced significantly as compared with an air burner. Thus it is difficult to provide adequate cooling of the burner using the oxidant.
Cooling of burners using oxygen or oxygen-enriched air is often provided by a separate cooling fluid. The most common cooling fluid is water. The amount of heat that cooling water is able to remove is a function of the conduction heat transfer from hot surfaces to water cooled surfaces and the convection heat transfer from water cooled surfaces to the water. It is generally desirable to provide cooling water as close to the hot surfaces as possible at a sufficient velocity to effectively transfer heat from the hot surfaces to water.
For a post-mixed burner having separate fuel and oxidant conduits which discharge into a furnace or a small burner block at the burner face, it is desirable that both the fuel and the oxidant conduits as well as the burner exterior surfaces be directly cooled by water in order to provide effective cooling.
One known method for providing cooling to a post-mixed burner is to provide cooling fluid in an incoming and outgoing annular stream between the fuel and annular oxidant conduits and in a separate incoming and outgoing annular stream on the outside of the oxidant conduit. Although such a system adequately cools the fuel and oxidant conduits and brings cooling fluid quite close to the burner face, it is disadvantageous because of the high fabrication costs required for the two separate cooling streams and also because it significantly increases the burner outside diameter.
Another known cooling system for a post-mixed burner which can be employed when the oxidant and fuel are delivered to the burner face in separate, i.e. not concentric, tubes employs a number of oxidant tubes submerged in cooling water. Such a system effectively cools the burner but has the disadvantages of high fabrication costs, especially when the number of oxidant tubes is large, such as greater than four, and of high pressure drop in the oxidant tubes because of the small total cross-sectional area of the oxidant tubes.
It is often desirable to outfit the discharge end of oxidant tubes with directional nozzles which direct the oxidant flow in a direction other than straight ahead. It is further desirable that such nozzles be replaceable to allow for a variety of flow directions. However, such replaceable nozzles require a solid portion proximate the burner face in order to provide a threaded seat to hold the nozzles. This portion is more susceptible to overheating because of its proximity to the burner face. The burner having a cooling system such as the one first described above is not applicable to this situation since it employs an annular oxidant tube. Replaceable nozzles are employed only on separate oxidant passages. The problem of cooling such a burner is made greater by the cooling requirements of the replaceable nozzles which can oxidize and seize in the threaded area thus rendering them incapable of removal.
It is therefore an object of this invention to provide a burner having an improved cooling system.
It is another object of this invention to provide a compact cooling system for a post-mixed burner which uses oxygen or oxygen-enriched air as the oxidant.
It is a further object of this invention to provide an effective cooling system for a burner which employs replaceable nozzles.